Parallel to their edges, photographic films, for example, incorporate one or two arrays of perforations which enable the film to be driven in devices for, exposing images onto and projecting images from the film, and in processing machines. In the use of photographic films, it is known that the quality of the projection, particularly the stability of the images on the screen, is closely dependent on the quality of the perforations, i.e. on the constancy of their pitch and on the accuracy of execution of each perforation. In this respect, the tolerance required is extremely fine, in the order of only a few microns on the finished perforation.
The accuracy of execution of the perforations depends on the accuracy of the movement of the punches at the time of perforation, though it also depends on the accuracy of mounting said punches.
In reciprocating movement type perforators, the punches are generally mounted on a block of a suitable thickness. The block is fastened to the upper jaw of the perforating device, and it is arranged so that the punches cooperate periodically with a die block held on the lower jaw of the perforating device.
FIG. 1 represents a punch block support am on which the punches are mounted as in the earlier technique. The block represented comprises an array composed of 15 punches 1. The arrangement of these punches is represented in greater detail in FIG. 2. Each punch is housed in a recess 2 of suitable size and shape to receive on the one hand the punch 3 and on the other hand a positioning wedge 4 arranged so as to press on two adjacent faces of punch 3.
Wedge 4 is arranged so that its axis 5 forms an angle of approximately 45.degree. in relation to the alignment axis 6 of the punches. Recess 2 is extended along axis 5 so as to receive a screw 7 intended to press on wedge 4, thereby immobilising punch 3 at the same time along alignment axis b of said punches and along an axis perpendicular to the alignment axis. According to this technique, the positioning of the punches is adjusted individually. The drawbacks of such a technique are mainly as follows :
when one of the screws associated with one of the punches is loosened, the punch is no longer held in position; PA1 such an arrangement of the positioning wedges involves the screws associated with two adjacent punches being relatively close to each other and this can prevent the use of screws of sufficient size to achieve the required tightening; PA1 the shape of the recesses intended to receive the punches and the positioning wedges is relatively complex to achieve and involves fairly long construction times. PA1 immobilisation of the punches translationally in a perpendicular direction to the alignment axis of said punches, as well as in their own longitudinal direction, is realised by means of a screw associated with each of the punches of the array; and PA1 immobilisation of the punches translationally in the direction of the alignment axis of the punches is achieved by means of a narrow slot across the whole block thickness, the slot extending along the entire length of the punches. The slot extends, at each of its ends, perpendicular to the alignment axis of the punches, in order to define a longitudinal bar and two portions of transversal bars, the slot being located so that the distortion of the longitudinal bar brings relatively closer the portions of transversal bars so as to immobilise the punches. PA1 the punches are immobilised translationally, in a perpendicular direction to the alignment axis of the punches, as well as in their own longitudinal direction, by means of a screw associated with each of the punches of the array; PA1 b) the punches are immobilised translationally in the direction of the alignment axis of the punches by means of a narrow slot across the whole punch block support arm thickness, the slot extending on the whole length of the punch array. The slot extends, at each of its ends, perpendicular to the alignment axis of the punches, in order to define a longitudinal bar and two portions of transversal bars, the punches being immobilised by a distortion of the longitudinal bar, which distortion brings relatively closer the portions of transversal bars.